Underwater Stern Thruster Removal and Reinstallation

Posted by Michelle Howard

Tuesday, November 12, 2013

Hydrex removed the stern thruster of a 69-meter offshore supply vessel and installed a new unit during the vessel’s stop in Limbee, Cameroon. By carrying out both parts of the operation underwater the ship could continue its commercial activities and did not have to go to drydock.

The removal and reinstallation took place in September, but earlier a Hydrex diver/technician team performed a full underwater inspection of the stern thruster unit. This was done during a previous stop in Cameroon. The inspection revealed that the thruster was damaged too severely for an on-site repair and needed to be replaced with a new one.

The inspection allowed the Hydrex technical department to prepare every step in detail to make sure that the team could carry out both the removal of the old stern thruster unit and the installation of the new unit during a single operation.

When the vessel was sailing towards Limbee again at the end of August, a team mobilized to the ship’s destination together with all the necessary equipment. They set up a monitoring station next to the berthed ship.

One by one the diver/technicians then detached the blades and replaced them with blind flanges to prevent oil from leaking from the thruster. In the meantime, initial preparations were made in the stern thruster engine room for the removal of the unit so that there would be no ingress of water once the unit was taken out.

The team then disconnected the stern thruster from the thruster room and carefully lowered it in the tunnel. The divers extracted it from the tunnel and brought it to the surface. Simultaneously the team installed a blind flange to seal off the thruster tunnel from the engine room.

For ease of handling, the new stern thruster unit was put on a cradle. The divers then lowered it into the water and brought it inside the tunnel.

Next the diver/technicians sealed off the thruster tunnel with the Hydrex flexible mobdocks and emptied all water from it. This created a dry working environment in which they could complete the reinstallation of the stern thruster unit in the best possible conditions. They repositioned the gearbox using chain blocks and secured it with bolts. The thruster propeller blades were then reinstalled one by one and the thruster unit was reconnected to the engine room.